Disclaimer
In a period of about 2 years, a product that had been in dominant use for 40 years was relegated to a lesser role in the industry. The book is a review of current work and thinking and contains both the latest understanding of the complex process of admixture-cement interactions and the techniques that enable improvements in the performance of concrete.
Acknowledgements
Water-reducing agents
Background and definitions
The other members of the water-reducing mixture group possess a different function that cannot be achieved by mixture design considerations. A special type of accelerating water-reducing mixture is finding increasing use and is known in North America as a 'mid-range water-reducing mixture'.
The chemistry of the water-reducing admixtures
Thus, in the manufacture of normal water-reducing additives, a small amount of an air-removing agent may be added. In the case of materials with a higher sugar content, this is used to prepare the retarding water-reducing mixtures, which allow a longer time.
The effects of water-reducing admixtures on the water–
The salts can therefore only be present to produce normal water-reducing mixtures at low doses and to slow down water-reducing mixtures at higher doses. The three categories of key ingredients discussed above for the formulation of water-reducing mixtures account for the majority of commercially available products, but there may be limited use of insitol [28], polyacrylamide [29], polyacrylic acids [30 ] and polyglycerol [ 31 ].
The effects of water-reducing admixtures on the properties of concrete
The subsequent hardened state during the life of the structure where the concrete material must fulfill its structural or aesthetic role without deterioration. It is important that the durability of the concrete is not adversely affected by the presence of a water reducing admixture.
The effects of water-reducing admixtures on the properties of plastic concrete
In these cases, the amount of water required to correct the loss of slump is reduced in the presence of a water-reducing mixture. The most common application of water-reducing admixtures is to enable a reduction in the water-cement ratio while maintaining initial workability compared to a similar concrete containing no admixture. It appears that the water reducing admixtures are most effective at an aggregate to cement ratio in the range of 6.5-7.0 in these mixtures.
The higher the required workability, the greater the reduction in water-cement ratio when a water-reducing admixture is added. Aggregate types have previously been shown not to significantly affect the performance of water-reducing admixtures.
The effects of water-reducing admixtures on the properties of hardened concrete
The compressive strength at 28 days of concrete containing lignosulfonate and hydroxycarboxylic acid water-reducing admixtures is a function of the water-cement ratio and follows Abram's rule v. Water reducing agents can be used to reduce the water-to-cement ratio to ensure minimum permeability. This work showed that by reducing the cement content in the presence of a water reducing agent, durability in the presence of sulfate solutions.
In view of this, it is concluded that both lignosulfonate and hydroxycarboxylic acid water reducing agents can be used to reduce the water-cement ratio of concrete mixtures, which would be reflected in an improvement in durability against sulfate attack. The effect of all types of water-reducing admixtures under these conditions is invariably to increase the shrinkage and creep of the concrete.
Proceedings of the International Symposium on Pore Structure and the Properties of Materials, RILEM/IUPAC, 5, 167–76. Proceedings of the Workshop on the Use of Chemical Admixtures in Concrete, University of New South Wales, 43–73. Proceedings of the Workshop on the Use of Chemical Admixtures in Concrete, University of New South Wales, 149–76.
Superplasticizers
Background and definitions
The chemistry of superplasticizers
This raw material was one of the first materials referred to in the literature as a water reducing agent [16], but only since 1970 has it been extensively used in blend formulations. The value of n is typically low, but conditions are chosen to obtain a higher molecular weight proportion of product as this is believed to be more effective [18]. The length of polymerization time will affect the molecular weight of the product, with the most useful average molecular weight being approximately 30,000.
When used alone, it is characterized by having minimal effect on air entrainment or curing time. The various types of polyacrylate polymers are prepared from the relevant monomers by a free radical mechanism using peroxide initiators and can be "block" or "random" polymers depending on the degree of prepolymerization of the monomers used.
Effects on the water–cement system
At a fixed level of SNF addition, the Blaine surface area of cement is directly proportional to the apparent viscosity [30]. The zeta potential of polyacrylate-based materials is significantly lower than that of SNF or SMF as shown in Fig. In studies with SNF [38] of different molecular weight, there is an indication that the higher molecular weight fraction causes less retardation of the hydration process than the lower molecular weight materials.
The type and amount of sulfate and alkalis affect the degree of retardation of the C3S phase. The polyacrylate materials are similarly adsorbed and cause dispersion of the cement particles partly by the same electrostatic mechanism through the ionized carboxyl groups.
Effects of superplasticizers on the properties of concrete
In the case of SNF and SMF, these are the ionized sulfonate groups on the adsorbed superplasticizer molecules. There is some delay in cement hydration, but after 28 days the products of C3S hydration are essentially the same as in a non-superplasticized cement system.
The effects of superplasticizers on the properties of plastic concrete
Full-scale trials conducted with a polyacrylate type superplasticizer on ready-mixed concrete [57] indicated that the specific type of material evaluated produced a very stable air void system, as shown in Table 2.3. There is an indication that the higher-C3A content cements lead to faster slump loss of superplasticized concrete [61]. It has been suggested that this is due to this type of superplasticizer operating through the 'steric hindrance' mode rather than through electrostatic repulsion [63].
The numbers on the slump-loss curves indicate the percentage of superplasticizer active ingredient by mass of cement. The effect that superplasticizers have on the setting time of concrete depends on a number of factors, including the type of superplasticizer, cement composition, and especially whether there is a simple addition of the admixture to the concrete or as a reduction in water-cement ratio is. made.
The effects of superplasticizers on the properties of hardened concrete
When superplasticizers are used to reduce the water content of the concrete mix, the increase in strength is normally considered to follow that of Abram. Most studies in this field have shown that superplasticized concrete has shrinkage and creep. In the early days of the use of superplasticized concrete, some concerns were raised regarding the resistance of air-entrained concrete containing superplasticizers to freeze-thaw cycling.
Physical Properties of Concrete 2000, Dhir, R.K. Chemistry and Dispersing Performance of Concrete 2000, Dhir, R.K. Proceedings of the Workshop on the Use of Chemical Admixtures in Concrete, University of New South Wales, 153. Proceedings of the First International Congress on Polymer Concretes, London, 6. Concrete Rheology, Materials Research Society Symposium.
Air-entraining agents
Background and definitions
The presence of a small amount of entrained air (2-4% by volume) leads to an improvement in cohesion or mix stability. Alternatively, with mixtures that are sufficient in this respect, a reduction in the sand content can be made when air is entrained without loss of cohesion. The amount that can be removed is approximately the same on a volume basis and leads to a reduction in the water-cement ratio to minimize the effect of entrained air on compressive strength.
When air entrainment is used in the above broad applications to achieve a given end result, there are a number of side effects that must be taken into account: (1) the presence of microscopic air bubbles acts as a 'lubricant' and increases the workability, or allowing a consequent reduction in the water-cement ratio;.
The chemistry of air-entraining agents
In order to meet the requirements of both in-use performance and the ability to form stable aqueous solutions of sufficient strength, the fatty acids shown in Table 3.1 are used as air entraining agents for concrete in the form of their alkali metal salts. These fatty acids are present in a range of chain lengths in naturally occurring fats and oils, such as tall oil and coconut oil and are used in the form of such a mixture. The common raw material is orthododecylbenzenesulfonate, which is a basic surfactant used in a variety of industrial and household detergents.
These products are also compatible with many water reducers to produce air-trapping water reducers. Although the vast majority of commercially available air barrier agents are simple solutions of materials within one of the above categories, it is possible to produce mixtures and this is occasionally done.
The effects of air-entraining agents on the water–cement system
It can be seen that the magnitude of the effect due to the presence of the additive itself is small in relation to the effect of the air it causes to be entrained. It can be concluded that the degree of air entrainment achieved in cement paste of a given consistency is largely a function of the type and dosage of the air entraining agent incorporated, while other variables have only a minor effect . A study of the foaming capacity and stabilities [10] of a variety of air-entraining agents in a solution of cement extracts showed that commonly used anionic air-entraining agents, such as sodium dodecyl sulfate and sodium resinate (1) were visually precipitated from solution, ( 2) retain their ability to mimic stable foam.
The solubility of calcium dodecyl sulfate is approx. 0.1 g1−1, so it is indicated that the optimum value lies in the range 0.1-0.5 g1−1 solubility of the calcium salt. There is no evidence to suggest that the presence of air entraining agents of the type normally available commercially in any way alters any hydration products of the cement.
The effects of air-entraining agents on the properties of plastic concrete
The properties and amount of cement used in the production of air-entrained concrete can have a pronounced effect on the air content and/or. The fineness of cement is an important factor in determining the amount of air entrainment required to contain a given amount of entrained. An investigation [21] of mortars containing cement differing only in their alkali content indicated that when the alkali (as Na2O) in the water in contact with the cement reaches about 0.8% by weight of water, the amount of air entrainer required to achieve a given air content is minimized.
The temperature of the concrete has a significant influence on the amount of air entrained in concrete by a. When fine particles with a diameter of less than 20 µm are included in the mixture design, the amount of air entrainment agent must be increased to obtain the required air content [ 25].
